This invention relates to a battery separator and, more particulary, to such a separator which includes at least one non-woven sheet of organic fibers, particularly polyolefin fibers, and may include, in addition, at least one non-woven sheet of glass fibers.
Battery separators which are non-woven sheets of polyolefin fibers are known, having been disclosed in a paper presented at a Power Sources Symposium in 1986 and published in a journal article, Akira Kitas, Seiji Yoshida and Shiro Tanso, xe2x80x9cCharacteristics of Large Sealed Nixe2x80x94Cd Batteries with Grafted Nonwoven Fabrics Separator,xe2x80x9d pp. 413-419 and in U.S. Pat. Nos. 5,922,417 and 5,830,604, granted Jul. 13, 1999 and Nov. 3, 1998 to Singleton et al. These references disclose that the hydrophobic nature of polyolefin fibers makes sheets thereof unsuitable for use as battery separators, and a treatment to make the surfaces hydrophilic by graft polymerizing acrylic acid or the like to the surfaces. The Kitas et al. reference disclose the use of an electron beam to cause the graft polymerization, while Singleton et al. disclose the use of benzophenone as a photoinitiator and UV irradiation for that purpose.
The Instant Invention
The instant invention, in one aspect, is based upon the discovery of an improved method for providing a hydrophilic surface on polyolefin fibers. A non-woven sheet of the fibers with the hydrophilic surfaces can be used as a battery separator. The improved method involves treating the polyolefin fiber surfaces by high energy discharge, e.g., with plasma glow discharge, flame plasma, or an electron beam, coating the treated surfaces with a vinyl monomer such as acrylic acid or with a combination of vinyl monomers such as acrylic acid and triallyl cyanurate, and polymerizing the vinyl monomer or combination of vinyl monomers on the surfaces of the polyolefin fibers. The coating step can be carried out by dipping, printing, vapor deposition, or the like, while the step of polymerizing the vinyl monomer or the combination of vinyl monomers can be carried out using heat, UV irradiation, an electron beam, plasma glow discharge, flame plasma or the like.
In another aspect, the invention is based upon the discovery that a polymeric coating which is hydrophilic in nature can be produced on the surfaces of polyolefin fibers to make the surfaces hydrophilic so that a non-woven sheet of such fibers can be used as a battery separator.
In yet another aspect, the invention is based upon the discovery that polyolefin fibers can be coated with a mixture of a hydrophilic monomer and a copolymerizable acid monomer and the mixture can be reacted to form a grafted copolymer or to form a copolymer coating on the fibers.
In still another aspect, the invention is based upon the discovery that the copolymerizable acid can be a sulfonate, so that the grafted polymer or coating produced on the surfaces of the polyolefin fibers has highly specific properties.
In yet another aspect, the invention is based upon the discovery of a battery separator comprising at least one layer of a fabric containing polyolefin fibers, from 1 percent by weight to 80 percent by weight of which are dividable fibers composed of polypropylene segments and polyethylene segments, and from 10 percent by weight to 50 percent by weight of which are sheathcore fibers having a polypropylene core and a polyethylene sheath, the surfaces of the polyolefin fibers are hydrophilic as a consequence of plasma discharge treatment which occurs upon ionization of a suitable atmosphere, i.e., one containing SF6, CHFCl2, oxygen, argon, or the like. The separator also comprises at least one layer which is a wetlaid or drylaid glass fiber sheet, a porous polymer film, a meltblown web of polymer fibers, a drylaid web composed of glass fibers and polymer fibers, a woven glass fiber web or a woven polymer fiber web. The multiple layers are chemically bonded or heat bonded together, or are mechanically entangled or hydroentangled.